The present invention relates to a circuit arrangement for the electro-thermal measurement of the filling level in the tank of a motor vehicle which includes a resistance sensor heated by a constant current and a sequence control for the cyclic operation of an evaluation circuit, whereby the resistor sensor is adapted to be heated during a sampling period and the voltage or current values are measured at the beginning and at the end of the heating period of the resistance sensor for indicating the filling level of the tank.
A circuit arrangement is already known (DE-OS No. 33 37 779), in which the filling level similar as with a heat wire probe is determined by means of an electro-thermal immersion tube generator. An ohmic resistance sensor immersed in the tank is Periods--typically 20 seconds to 60 seconds--for a predetermined heating period of typically 0.5 seconds to 2 seconds and the voltage or the current at the beginning and at the end of the heating period is measured and the change thereof is correspondingly evaluated. In a motor vehicle, the first measurement takes place after a predetermined time of about 5 seconds after the starting of the engine. This time period is necessary in order to assure a sufficiently high voltage supply after the engine start and is selected as brief as possible in order to be able to indicate as rapidly as possible the actual filling level. The time interval (sampling period-heating period) between two measurements, by contrast, is necessary in order to cool off the resistance sensor again after a load (cool-off period).
If the engine is now successively started several times within a sampling period, the resistance sensor can no longer cool off which leads to an undesired temperature rise and therewith to error measurements.
It is therefore the object of the present invention to so construct a circuit arrangement that an undesired temperature increase in the resistance sensor and therewith error measurements are avoided.
The underlying problems are solved according to the present invention in that the sequence control electrically connected with the voltage supply is electrically connected with a further voltage source from which, after the turning-off of the voltage supply, the sequence control continues to be supplied with voltage by way of switching means over a time interval corresponding to the end of the sampling period and which after this time interval is automatically turned off by the switching means.